1. Field of the Invention
The present invention relates to the continuous measurement or monitoring of the cardiac output of the human body. Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in critically ill patients, patients with suspected cardiovascular and pulmonary disease, and high risk patients undergoing surgery. The present invention also relates to continuous monitoring of oxygen transport. Oxygen transport is the volume of oxygen transported from the heart and lungs to the body per unit time. It is useful to assess the cardiorespiratory status of the above patients.
2. Description of the Prior Art
Cardiac output has been measured by a number of different methods. Several methods are described in Cardiac Output Measurements. A review Of Current Techniques And Research, by Ehlers et al. in the Annals Of Biomedical Engineering, Vol. 14, pp. 219-239, 1986. This publication discusses both "intermittent" cardiac output measurements for obtaining a single measurement and "continuous" measurements in which various patient parameters are continuously monitored and cardiac output calculated on a regular and repeating basis. Although high accuracy can be obtained with certain intermittent measurement techniques, it is very desirable to be able to provide continuous information regarding cardiac output. Currently, the most popular technique for measuring cardiac output intermittently is via an indicator dilution method, and particularly thermodilution. In indicator dilution techniques, a predetermined amount of substance is introduced at a single point in the bloodstream and analyzed at a point downstream to obtain a time dilution curve. The average volume flow is inversely proportional to the integrated area under the dilution curve. In the thermodilution method, the indicator is a temperature change of the blood. The temperature change is typically produced by injecting cold saline through a catheter into the right atrium. This results in a cooling of the blood, which is measured at a downstream location with the same catheter to produce a thermodilution curve. Cardiac output can then be determined. This technique employs a catheter with a thermistor at the tip.
Other methods of cardiac output measurement are based upon the Fick principle. According to this principle, the rate of uptake or release of a substance to or from blood at the lung is equal to the blood flow past the lung and the content difference of the substance at each side of the lung. This can be expressed by the equation: EQU Uptake =Q (c.sub.2 -c.sub.1),
where Q is the blood flow (cardiac output), c.sub.2 the content of the substance leaving from the lung and c.sub.1 the content of the substance coming to the lung. Applying this relationship to oxygen yields: EQU Q =VO.sub.2 /(c.sub.a O.sub.2 -c.sub.v O.sub.2), (1)
where VO.sub.2 is the volume of oxygen inspired per unit time and c.sub.a O.sub.2 and c.sub.v O.sub.2 are respectively the arterial and mixed venous oxygen contents. Applying the relationship to carbon dioxide yields: EQU Q =VCO.sub.2 /(c.sub.v CO.sub.2 -c.sub.a CO.sub.2), (2)
where VCO.sub.2 is the volume of carbon dioxide produced by the patient per unit time and c.sub.v CO.sub.2 and c.sub.a CO.sub.2 are respectively the mixed venous and arterial carbon dioxide contents. Determination of VO.sub.2 and VCO.sub.2 require a volume measurement (e.g., via integration of a flow signal or via a rotameter) and a fractional concentration measurement (e.g., via mass spectrometer or gas analyzer (infrared or polarographic)).
The Fick method is most commonly used with oxygen as the analyzed substance. Equation (.sub.1) has been used to obtain intermittent measurements of Q. Via indwelling catheters, arterial and venous blood samples were obtained and these samples were analyzed on a blood gas analyzer to obtain the oxygen saturation (SO.sub.2) and the partial pressure of oxygen (PO.sub.2). Arterial and venous oxygen contents were then calculated from the formula: EQU cO.sub.2 =1.34.Hgb.SO.sub.2 +0.0031.PO.sub.2, (3)
where Hgb is the hemoglobin in gm/100ml of the patient, SO.sub.2 is in percent, cO.sub.2 is in ml of O.sub.2 /100ml of blood, and 1.34 is a constant in ml/gm (other values of this constant, e.g., 1.36 and 1.39 have also appeared in the literature). The above approach only yields intermittent measurements and is also cumbersome. Therefore, it is typically not used in critically ill patients.
The dissolved oxygen (0.003l.PO.sub.2) in equation (3) is generally negligible so that equation (1) can be simplified to: EQU Q =VO.sub.2 /(13.4.Hgb.(S.sub.a O.sub.2 -S.sub.v O.sub.2)) (4)
where Q is in 1/min, VO.sub.2 in ml/min, Hgb in gm/100ml and S in percent. S.sub.a O.sub.2 can be measured continuously via an oximeter (pulse, transmission or indwelling type). Similarly, S.sub.v O.sub.2 can be measured continuously via reflectance oximetry and a fiberoptic pulmonary artery (right heart) catheter. Several systems have been developed to continuously monitor cardiac output via continuous measurement of VO.sub.2, S.sub.a O.sub.2 and S.sub.v O.sub.2. Such methods are described in Hankeln, et al. Continuous, On-line, Real-Time Measurement of Cardiac Output and Derived Cardiorespiratory Variables in the Critically Ill. Crit. Care Med 13, 1071, 1985; Davies, et al. Continuous Fick Cardiac Output Compared to Dilution Cardiac Output. Crit. Care Med 14, 881, 1986; and Tachimori, et al. On-line Monitoring System for Continuous and Real-Time Cardiac Output. Crit Care Med (Abstract) 14, 401, 1986. The methods described provide fairly good results; however, oxygen Fick methods all have the common drawback that it is difficult to measure the body's rate of oxygen uptake accurately. This is particularly so when the patient is inspiring a high concentration of oxygen (FIO.sub.2), as occurs frequently in critically ill patients. In addition, FIO.sub.2 can vary from breath to breath in patients on ventilators (this occurs because of inaccuracies in the internal blender or pressure fluctuations in the oxygen and air supply). VO.sub.2 measurement is therefore difficult in ventilator dependent patients unless a blender external to the ventilator or calibrated gases from an external tank are used. Furthermore, patients may be on various modes of ventilation such as flow-by, where part of the oxygen bypasses the patient's mouth. In this situation, complicated valving is required to separate the patient's exhaled gas from the flow-by gas. It is therefore difficult to provide a universally applicable system for VO.sub.2 measurement.
The difficulty of VO.sub.2 measurement at high oxygen concentration is recognized in the Davies et al publication and is theoretically discussed in Ultman, et al., Analysis of Error in the Determination of Respiratory Gas Exchange at Varying F.sub.I O.sub.2, J Appl Physiol 50, 210, 1981. The Davies publication mentions approximating the VO.sub.2 by measuring carbon dioxide output (VCO.sub.2) and dividing it by an assumed respiratory quotient RQ. This method has the potential disadvantage that the assumed value of the respiratory quotient of the patient may be incorrect.
The Fick method has also been applied to carbon dioxide employing equation (2) to obtain intermittent cardiac output. The c.sub.v CO.sub.2 is usually estimated from the partial pressure of carbon dioxide (P.sub.v CO.sub.2). The latter may be obtained indirectly by breath holding or more popularly by rebreathing. Such methods are described in Davis, C.C., et al. Measurements of Cardiac Output in Seriously Ill Patients Using a CO Rebreathing Method. Chest 73, 167, 1978; and Blanch, et al., Accuracy of an Indirect Carbon Dioxide Fick Method in Determination of the Cardiac Output in Critically Ill Mechanically Ventilated Patients, Int. Care Med 14, 131, 1988. A major disadvantage of this method is that it yields only intermittent values of Q since the partial pressure of carbon dioxide is estimated via rebreathing. More recently, a partial rebreathing method that does not require monitoring of P.sub.v CO.sub.2 has been used in dogs (Capek, et al., Noninvasive Measurement of Cardiac Output Using Partial CO.sub.2 Rebreathing. IEEE Trans. Biomed. Eng. 35, 653, 1988). This method is also intermittent and it is unlikely that it can be easily applied to patients with lung disease.